1. Field of the Invention
This invention relates to voltage transient protection circuits and more particularly to a voltage transient suppressor circuit to protect integrated circuits from high negative transient voltages.
2. Description of the Prior Art
Hybrid circuits or subscriber loop interface circuits (SLICs) are known in the art which provide, for example, signal conversion between a balanced two-wire, bidirectional subscriber loop and a pair of unidirectional transmission paths. SLICs are required for connecting conventional telephone sets to telephone lines from electronic switching systems such as private automatic branch exchange (PABX) systems or telephone central office (CO), level five, switching circuit systems.
A conventional integrated circuit SLIC is manufactured by Motorola Inc. and is sold under the MC3419/3519 product number. The MC3419 is shown and described on page 6-30 in the Motorola Inc. "Linear Interface Integrated Circuits" Data Book, series C, copyrighted 1979. As described, the MC3419 SLIC is protected from high voltage transients occurring at the tip and ring terminals by a diode bridge coupled in conjunction with a pair of series current limiting resistors. These high voltage transients are created by lightning striking near telephone cables. Lightning created voltage transients are generally specified as being equal to a negative 1500 voltage spike of generally two milliseconds duration.
A problem that occurs with the aforementioned diode bridge, which will be more fully explained later, is that the negative voltage transients are caused to be shorted to the main minus 48 volt battery located at the central office. Thus, although the SLIC is protected therefrom, due to the two limiting resistors which are connected respectively to the tip and ring terminals in conjunction with the diode bridge, depending upon the number of telephone cables (sometimes upwards to 100 or more) thousands of amperes of current could cause to be discharged through this battery. Needless to say this is highly undesirable.
Another method utilized in the prior art for transient protection is to utilize a pair of fifty-volt Zener diodes coupled to the respective tip and ring lines. This method would work quite well except that the minus 48 volt battery located at the central office can actually vary in its output from between a minus 42 to minus 56 volts. Thus, if the Central Office battery is greater than minus 50 volts, the zener diodes are on all the time dissipating power and if the battery output is less than minus 50 volts, it is possible to damage the SLIC.
Thus, there is a need for a high voltage transient suppressor circuit suitable for use with SLICs to protect both the SLICs and the central office equipment from damage which could be otherwise caused by the high voltage transients.